In 1803, English Chemist John Dalton was able to conduct an experiment that proved all elements are composed of small indivisible particles called atoms.
Specifically, Dalton’s theory specified:
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
In chemical reactions, atoms are combined, separated, or rearranged.